The invention refers to an apparatus for pre-treatment and subsequent plastification or agglomeration of synthetic plastic materials, in particular thermoplastic waste plastics for recycling purposes, comprising a receptacle that has an intake opening for the material to be processed and in which at least one circulating comminuting and/or mixing tool is disposed, whereby the material processed within the receptacle is fed into the housing of a screw plasticizing or agglomerating this material, for example an extruder screw, which housing preferably is connected to the receptacle.
Apparatus of this kind are known in many embodiments. The pre-treatment of the synthetic plastic material to be processed mostly is a comminuting action which, however, can be also replaced by or can be combined with mixing and/or drying and/or heating and/or crystallizing and/or densifying of the material. In most apparatus of this kind, the tools revolving within the receptacle press the pre-processed material directly into the housing of an extruder screw, which housing is connected to the receptacle. However, apparatus are also known in which the material leaving the receptacle first enters a tube in which a conveying screw may be disposed. The material fed by this horizontally or vertically directed tube then reaches the housing of an extruder screw by which the material is finally plasticized.
The screw length of a usual plasticizing screw can roughly be subdivided into three zones: namely the intake zone neighboring the intake opening, then the compression or melting zone following the intake zone, and lastly the subsequent dosing discharging or metering zone. Usually, the depth of the volutions of the screw in the metering zone is less than the depth of the volutions in the intake zone, whereby the material being still loosely within the intake zone, for example material in the form of flakes, is more and more compressed over the course of the screw length. For processing of commercial plastics, for example polyethylene, commercial extruders have a depth of the volutions in the metering zone of about 5% of the screw diameter.
Most of these known constructions do not meet the requirements with respect to the quality of the processed synthetic plastic material obtained at the exit of the screw and/or with respect to the yield of the screw. Investigations have shown that the requirements to the screw following the receptacle, mostly a plasticizing screw, are not constant during the operation and that this can be explained by the fact that some portions of the processed material remain for a longer time within the receptacle than other portions. The mean dwell time of the material within the receptacle can be calculated by the filling weight within the receptacle divided by the output of the screw per time unit. This mean dwell time, however, as already mentioned, is as a rule not fulfilled for big portions of the material to be processed, but there occur irregular substantial positive and negative deviations from this mean value. These deviations can be caused by different properties of the material batches introduced step by step into the receptacle, for example different quality or different thickness of the synthetic plastic material, for example foil rests and the like, however, also by uncontrollable random events.
For thermically and mechanically homogeneous material an improvement of the quality of the material obtained at the exit of the screw is attained if the depth of the volutions of the screw in the metering zone is very great and the revolution speed of the screw is kept very low. Investigations have shown that this can be explained by the fact that the processed material is subjected to a low shearing action by such a screw geometry. The shearing of a processed material (shearing speed) can be calculated by the speed of the periphery of the screw divided by the depth of the volutions of the screw. With such a screw geometry, the material is only slightly mechanically and thermically stressed so that the molecule chains of the synthetic plastic material are not or not substantially adversely affected.
If, however, it is important to increase the output of the screw or, for example, the performance of a shredder-extruder combination, then the revolution speed of the screw must be increased, and this means that also the shearing effect is increased. However, the processed material is thereby subjected to higher mechanical and thermical stresses by the screw; i.e. there is the danger that the molecule chains of the synthetic plastic material are adversely affected. As a further disadvantage, there occurs more wear of the screw and its housing, in particular when processing recycling material due to the impurities contained within this material, for example abrasive particles, metal pieces or the like, which have a high wear effect on the metal elements of the screw and its bearings sliding along each other.
With a slowly rotating and deeply cut screw (great depth of the volutions) as well as with a quickly rotating screw, the already mentioned different quality of the single material batches supplied to the screw, for example different flake sizes and/or different temperature of the synthetic plastic material, have an adverse effect in view of inhomogeneities of the synthetic plastic material at the screw exit. In order to equalize these inhomogeneities, in practice, the temperature profile of the extruder is increased, and this means that additional power must be supplied to the synthetic plastic material, which has as a result the mentioned thermic damages of the synthetic plastic material and an increased power consumption. Further, the viscosity of the plastic material obtained at the extruder exit is reduced thereby, so that this material is very fluid, which causes difficulties when further processing this material.
Therefrom it can be seen that the processing parameters favorable to obtain a good material quality at the exit of the screw are in contradiction to each other.